I think the old adage always applies when buying a tool, especially a machine. Get the most you can afford.
You don't need a vs lathe. You'll still be able to produce the same quality pens as those with vs. But if you do get one I don't think you'll regret it, unlike you might with certain other upgrades that are available that you may never use.
When it comes to pen making I only change speeds when moving between turning and polishing.
Cheers
Ash